Rear brake question 85 K100RS

diversity48 · 1 Rear brake question 85 K100RS Tue Apr 21, 2015 4:07 am

diversity48

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Number of posts : 22

After bleeding the front brakes succesfully, I tried bleeding the rear. Seems that I have a bad Master cylinder, here is what it does: Removed most of the fluid from the reservoir, filled it with fresh fluid, and proceeded to drain the caliper using a "Mity-Vac" which I have used on other bikes and on the front calipers with complete success. Only could get a few drops into the canister of the mity-vac at first, and after pumping it several times, ended up with only a very small amount of fluid there...possibly 1/2" or so in the bottom of the canister. Tried bleeding the old fashioned way...pumping the pedal, and after MANY pumps, probably 100-150 or so, finally got the rear brake to function, but just barely. Now I have very little rear brake power. After reading related posts on the subject, I'm thinking that it may be the master cylinder. With these symptoms, what would your opinion be? I have not taken the caliper off, but I'm thinking that even with leaky seals, the caliper should at least be kicking out some fluid from my efforts. Just need some educated guesses here...what do you think?

KafeRacer · 2 Re: Rear brake question 85 K100RS Tue Apr 21, 2015 4:35 am

KafeRacer

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I have a rear master if you need one $50 usd delivered.

Cheers.

Point-Seven-five · 3 Re: Rear brake question 85 K100RS Tue Apr 21, 2015 4:42 am

Point-Seven-five

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Are you seeing the fluid level in the reservoir going down? If not, there may be a blockage in the supply from the reservoir to the master cylinder.

Is there fluid leaking out under the boot where the pedal pushes on the master cylinder piston. If yes, the piston seals are bad and you're pissing the fluid out on the floor.

Can you disconnect the brake line at the caliper and get fluid when the master cylinder is pumped? If yes, you may have a blocked bleeder.

diversity48 · 4 Re: Rear brake question 85 K100RS Tue Apr 21, 2015 5:31 am

diversity48

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Number of posts : 22

No leaking beneath the bike ... it would be obvious if it was leaking there. I pumped the hell out of the pedal and the reservoir on the bike remained at or very near the same level. Normally, with the bleeder system I have, it supplies enough "suck" to deplete the reservoir rather quickly. Not so in this case. The reason I'm suspicious of the master cylinder is that no, or at least very little fluid seems to be getting back to the caliper. It went from "no rear brake" to "very little" rear brake.

diversity48 · 5 Re: Rear brake question 85 K100RS Tue Apr 21, 2015 5:34 am

diversity48

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Oops...NO, I haven't tried disconnecting the hose at the caliper...good thought though...at least worth a try. Also could be the blocked bleeder, but if it were, wouldn't the brakes at least function?

brickrider · 6 Re: Rear brake question 85 K100RS Tue Apr 21, 2015 6:50 am

brickrider

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I replaced the line on the rear brake last year thinking a stainless steel one would firm up the pedal. No joy.
My bike is a 1985 K100RS too. It has only about 40K on the clock, but that spongy rear brake has me wondering if the m/c isn't K-putt. Does BMW sell rebuild kits for the master cylinder of the rear brake?

92KK 84WW Olaf · 7 Re: Rear brake question 85 K100RS Tue Apr 21, 2015 11:04 am

92KK 84WW Olaf

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brickrider wrote:I replaced the line on the rear brake last year thinking a stainless steel one would firm up the pedal. No joy.
My bike is a 1985 K100RS too. It has only about 40K on the clock, but that spongy rear brake has me wondering if the m/c isn't K-putt. Does BMW sell rebuild kits for the master cylinder of the rear brake?

I had same issue with my 92 K100LT rear brake. We got a rebuild kit for the master cylinder and it did the business. also did the 84 K100RT last April. Rebuild kits used in both cases but the RT needed a replacement reservoir [cheap] and also needed the pistons and caliper seals sorted. Pistons were sticky giving no real braking pressure.

Dai · 8 Re: Rear brake question 85 K100RS Wed Apr 22, 2015 6:01 pm

Dai

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I suspect a rebuild will be necessary. From your description I'm pretty sure that the feed hole in the m/c is blocked, so no (or very little) fluid is getting into the brake line. A blocked return hole will give similar symptoms.

What doesn't help is that BMW have a rubber block between the brake pedal and the m/c piston. This softens up the feel still further. My guess is they designed it in there to make locking up the rear wheel less of a problem. I threw mine away and replaced it with a length of stainless steel shaped to mimic the plate on the end of the rubber block. Much more feel and feedback.

While you're down there, pull the rear brake caliper off, remove the pads and scrub the guaranteed pile of crap out from the middle of the caliper. That will definately help with the braking. The dirt that gets in there can prevent the pads from moving smoothly; if it's bad enough I've seen it stopping the pads moving at all.

diversity48 · 9 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 4:08 pm

diversity48

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Guess I never really thought the caliper would be the suspect, but at least a :going through" on it would eliminate that. Gonna try that (hopefully today) first and see how far I get. Next would be either a new master or rebuilding the old one. Found a new one at bmw2valve.com for $169 that may be the ticket although more than I wanted to spend :-(...starting with the caliper first tho without pulling the trigger on the new m/c.

brickrider · 10 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 7:54 pm

brickrider

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Personally, I'm inclined to go to MotoBins for a rebuild kit. My bike seems to have been pretty well maintained by the previous owner(s) so I doubt the bore of the master cylinder is so pitted that a rebuild won't restore proper action.
And what's this about a rubber bit to soften the feel of the pedal?! I must have a closer look down there. That sure sounds like a daft idea to me, but then what do I know as compared to the BMW engineers? Shocked

sidecar paul · 11 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 10:03 pm

sidecar paul

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That black bit is the rubber part, Brickie......I tried replacing it with a solid stainless part, but I didn't like it (no 'feel' at all) so I added a 2.5 mm thick ring on the rubber to reduce the amount of squish. Works for me.

Paul.

Dai · 12 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 10:54 pm

Dai

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One thing to bear in mind when overhauling Brembo master cylinders (and someone here is really going to argue with me over this!); two overhauls and that's your lot. Shocked

Okay, so that's the factory statement Very Happy

Experience shows something different but there is a limit. When you've got the piston out, hold the body up to the light and check the insides very carefully. If it's nice and shiney and has wide silver streaks in there, introduce it to the bin. The surface anodising has worn off and the chances are even a new seal will either not work or will not work for very long.

Yes, I have fallen foul of this. The symptoms are an overhauled system that will bleed well but will not hold pressure. It happened to me about fifteen years ago and I ended up having a very nice chat with a Brembo rep who gave me the 'two overhauls' information. Unfortunately I had to buy my own replacement master cylinder Sad

brickrider · 13 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 11:24 pm

brickrider

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I've heard it said by some of the wise men in the airhead community that even one rebuild on those front brake master cylinders is a waste of time and money. I understood the reason was pitting of the bore; never thought of the possibility of them being anodized or otherwise finished in any way.
I like the idea of a ring to limit the expansion of the rubber bit in the m/c. I can see that having a lathe in the shed is better than a very good thing Smile

... but then I'd have to learn how to use it Razz

Last edited by brickrider on Thu Apr 23, 2015 11:25 pm; edited 1 time in total (Reason for editing : grammar)

92KK 84WW Olaf · 14 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 11:24 pm

92KK 84WW Olaf

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I have to agree with this.

Not a surprise I hope.

Dai · 15 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 11:44 pm

Dai

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brickrider wrote:I've heard it said by some of the wise men in the airhead community that even one rebuild on those front brake master cylinders is a waste of time and money. I understood the reason was pitting of the bore; never thought of the possibility of them being anodized or otherwise finished in any way.

I have to utterly disagree with that sentiment. AFAIK the only thing that could cause pitting would be poor maintenance allowing sh!t to get in and/or not changing the fluid often enough. In almost thirty years of overhauling Brembo master cylinders on Guzzis, Laverdas and BMWs I have seen two that were visually worn beyond use (including the one that caught me out) but never one that was pitted. As I'm only a home mechanic and not a bike pro, I'd guess I've done perhaps 30-40 master cylinders front and rear and maybe 60-80 calipers of the P08/F08 type.

SteveK1 · 16 Re: Rear brake question 85 K100RS Tue Apr 28, 2015 4:05 pm

SteveK1

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Dai wrote:

brickrider wrote:I've heard it said by some of the wise men in the airhead community that even one rebuild on those front brake master cylinders is a waste of time and money. I understood the reason was pitting of the bore; never thought of the possibility of them being anodized or otherwise finished in any way.

I have to utterly disagree with that sentiment. AFAIK the only thing that could cause pitting would be poor maintenance allowing sh!t to get in and/or not changing the fluid often enough. In almost thirty years of overhauling Brembo master cylinders on Guzzis, Laverdas and BMWs I have seen two that were visually worn beyond use (including the one that caught me out) but never one that was pitted. As I'm only a home mechanic and not a bike pro, I'd guess I've done perhaps 30-40 master cylinders front and rear and maybe 60-80 calipers of the P08/F08 type.

OK, so from what you are saying I cannot overhaul my front master-cylinder? I have a 16 valve K1 and I think it has an all-aluminum 20 mm MC. I rebuilt it previously with new piston and seals, honed the bore. Several months later it seized up the front calipers and pitched me off. Thinking it may have been not cleaned adequately I have re-honned it and cleaned it in a sonication bath. I was about to reassemble it. Are you saying I would be unwise to try using this and would do better to source a new MC body? Could one re-anodize the bore?

Last edited by SteveK1 on Tue Apr 28, 2015 4:11 pm; edited 1 time in total (Reason for editing : spelling)

Dai · 17 Re: Rear brake question 85 K100RS Tue Apr 28, 2015 7:46 pm

Dai

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Personally I'd be looking for a replacement master cylinder.

SteveK1 · 18 Re: Rear brake question 85 K100RS Tue Apr 28, 2015 8:34 pm

SteveK1

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Dai wrote:Personally I'd be looking for a replacement master cylinder.

Personally? Why? Because of the all aluminum versus steel cylinder? More information please.

Reading in the thread, it looked to me like the MC I have should be repairable.

https://www.k100-forum.com/t1301-brake-seizure

Crazy Frog · 19 Re: Rear brake question 85 K100RS Tue Apr 28, 2015 11:39 pm

Crazy Frog

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Time to give my one cent of advise and experience..

Have you tried to play with the adjustor screw on the MC (This is the link between the pedal and the MC- #12 on the first picture- and screw #3 on the second one)
If the travel of the piston is limited, the piston will never return to the position where the filling hole is cleared to let more brake fluid re-enter the MC.
It's worth playing with the adjustments before replacing the unit.

Dai · 20 Re: Rear brake question 85 K100RS Wed Apr 29, 2015 6:22 pm

Dai

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SteveK1 wrote:

Dai wrote:Personally I'd be looking for a replacement master cylinder.

Personally? Why? Because of the all aluminum versus steel cylinder? More information please.

Reading in the thread, it looked to me like the MC I have should be repairable.

https://www.k100-forum.com/t1301-brake-seizure

As you say, it's been honed twice, so I wouldn't trust a new seal kit to make a good seal in there. Perhaps I'm being too cautious but experience is a hard taskmaster and I'd rather be absolutely sure of my brakes. You could try an overhaul kit and find it will work perfectly; I'd also check the two holes very carefully when you have it apart.

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1 Rear brake question 85 K100RS Tue Apr 21, 2015 4:07 am

diversity48

2 Re: Rear brake question 85 K100RS Tue Apr 21, 2015 4:35 am

KafeRacer

3 Re: Rear brake question 85 K100RS Tue Apr 21, 2015 4:42 am

Point-Seven-five

4 Re: Rear brake question 85 K100RS Tue Apr 21, 2015 5:31 am

diversity48

5 Re: Rear brake question 85 K100RS Tue Apr 21, 2015 5:34 am

diversity48

6 Re: Rear brake question 85 K100RS Tue Apr 21, 2015 6:50 am

brickrider

7 Re: Rear brake question 85 K100RS Tue Apr 21, 2015 11:04 am

92KK 84WW Olaf

8 Re: Rear brake question 85 K100RS Wed Apr 22, 2015 6:01 pm

Dai

9 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 4:08 pm

diversity48

10 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 7:54 pm

brickrider

11 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 10:03 pm

sidecar paul

12 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 10:54 pm

Dai

13 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 11:24 pm

brickrider

14 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 11:24 pm

92KK 84WW Olaf

15 Re: Rear brake question 85 K100RS Thu Apr 23, 2015 11:44 pm

Dai

16 Re: Rear brake question 85 K100RS Tue Apr 28, 2015 4:05 pm

SteveK1

17 Re: Rear brake question 85 K100RS Tue Apr 28, 2015 7:46 pm

Dai

18 Re: Rear brake question 85 K100RS Tue Apr 28, 2015 8:34 pm

SteveK1

19 Re: Rear brake question 85 K100RS Tue Apr 28, 2015 11:39 pm

Crazy Frog

20 Re: Rear brake question 85 K100RS Wed Apr 29, 2015 6:22 pm

Dai

21 Re: Rear brake question 85 K100RS

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